anyone know where in the UK i can get a tripod ring for a 70-200 F2.8? i just bought a used one today for a bargain price but it came without the ring sigma sell it direct for £49.99 (outrageous!) but hopefully someone knows somewhere i can get it cheaper? edit: for those that use similar lenses, how often do you find yourself using the bush/ring? is it worth spending £50 (think that's around $90-100 US) on one?